A vehicle having an internal combustion engine includes a fuel tank that stores liquid fuel, such as gasoline, diesel, methanol or other fuels. The liquid fuel evaporates into fuel vapors that increase pressure within the fuel tank. Evaporation is caused by energy that is transferred to the fuel tank. Sources of energy include radiation (e.g. sun energy), convection, and conduction. Increased vapor pressure in the fuel system may effect the rate that vapor fuel is released into the atmosphere through a leak in the fuel system. Vapor leak diagnostic systems attempt to diagnose vapor fuel leaks.